Influence of ruminal protozoa on site and extent of starch digestion and ruminal fermentation.

Eight ruminally, duodenally, and ileally fistulated sheep were used to study the effects of protozoa on ruminal fermentation and starch digestion in the rumen and intestines. The experimental diet (75% grain DM basis) was based on a mixture of 67% high-moisture corn (HMC) and 33% dry-rolled grain sorghum (DRGS). Protozoa were reduced from 51,286 to 13,987 organisms/mL of ruminal fluid by feeding a diet (13 d) containing 9% tallow and monensin at 27 mg/kg (DM basis). Three sheep were defaunated by intraruminal infusions of lactic acid (20 to 30 g/d) and one sheep by washing the rumen and heating ruminal contents (60 degrees C for 20 min). No differences were observed in DM or starch intake, ruminal fluid pH, lactate concentration, or total tract starch digestibility. Molar proportion of acetate was reduced (48.6 vs 58.3%; P < .01) and propionate was increased (32.2 vs 23.4%; P < .03) by defaunation. Total VFA tended (P = .20) to be lower in faunated sheep (118.5 vs 139.2 mM). In situ rate of starch digestion of HMC (22.3 vs 14.7 %/h; P < .02) and DRGS (5.1 vs 3.3 %/h; P < .009) was increased by defaunation. Starch digestibility (percentage of intake) in the rumen was increased (P < .001) by defaunation (84.2 vs 93.7%) and was reduced (P < .002) in the small intestine (5.2 vs 13.6%). Defaunation increased (P < .03) amylolytic activity (.40 vs .17 units/mg of protein; P < .03) and osmotic pressure (321 vs 245 mOsm/kg; P < .006).(ABSTRACT TRUNCATED AT 250 WORDS)